Cement mill and method of starting same

ABSTRACT

This invention relates to grinding plants having a cement grinding mill and auxiliary mechanical and electrical control installations for internal water injection to cool the product ground in the mill and for humidifying ventilating air drawn through the mill by a fan. An electrostatic precipitator provides for dedusting of the air while means to control the passage of air through the plant in relation to at least certain conditions in the plant such as temperature and moisture content, results in improved operation of the plant. The invention also relates to a method of operating and of starting such grinding plants in operation with improved operational performance and a minimum of pollution contribution of dust to the adjacent atmosphere.

United States Patent Carlsen 1 1 Dec. 2, 1975 15 1 CEMENT MILL AND METHOD OF 2,932,460 4/1960 Petersen 241/54 X STARTING SAME 3,282,514 11/1966 Putz 241/17 Primary Examiner-Granville Y. Custer, Jr. Attorney, Agent, or FirmPennie & Edmonds [57] ABSTRACT This invention relates to grinding plants having a cement grinding mill and auxiliary mechanical and electrical control installations for internal water injection to cool the product ground in the mill and for humidifying ventilating air drawn through the mill by a fan. An electrostatic precipitator provides for dedusting of the air while means to control the passage of air through the plant in relation to at least certain conditions in the plant such as temperature and moisture content, results in improved operation of the plant. The invention also relates to a method of operating and of starting such grinding plants in operation with improved operational performance and a minimum of pollution contribution of dust to the adjacent atmosphere.

32 Claims, 1 Drawing Figure CONTROL COMPUTER AND RECTIFIER CEMENT MILL AND METHOD OF STARTING SAME BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to grinding plants having a grinding mill with means for internally injecting water to cool the product ground and for humidifying the ventilating air. The invention also relates to an improved method of operating such plants.

2. Description of the Prior Art Grinding plants of the type including means for injecting cooling water, are well known and widely used in mineral processing industries. They are particularly utilized in processes for straight grinding of cement clinker and the like. Grinding plants of this kind are also used in various modified combinations with classifiers in processes in which the ground product is separated into a final fine-ground product and a coarse fraction which may be returned to the inlet end of the mill. The present invention is applicable to straight-grinding plants as well as in such closed-circuit grinding plants.

In order to protect the machinery during the starting of operation of such plants, and in order to prevent over-filling of conveyors, collecting boxes, and the mill, etc., the various electrical components which control the plant are provided with interlocking circuits. Such interlocking circuits include relays, computers or the like to ensure that the machinery can only be started in a predetermined order, either by hand or automatically. For example, it is obvious that conveyors positioned after the mill in the circuit must be started before the mill is started, and that the mill must be started before the feeders begin to feed material into the mill. Auxiliary machinery such as pumps for delivering oil to lubricate bearings and compressors supplying air to air conveyors have, similarly, to be started before the main machinery is started. Such interlocking systems often include means for compulsorily stopping the plant or part thereof in case of failures occurring in the plant.

The starting procedures of present day plants have various drawbacks. For example, in such grinding plants dust settles in the machinery and in the pipes when the plant is not in operation. Thus, during the starting operation, the settled dust is whirled up with only a limited portion being caught by the electrostatic precipitator. As a result, part of the dust leaves the grinding plant through the air vent and causes pollution of the atmosphere in the building and its surroundings.

The present invention relates to improved grinding plants and an improved method of starting up such grinding plants. In particular, the invention relates to a grinding plant for grinding cement clinker and the like in which the dust nuisance is either completely eliminated or, at least, reduced to an acceptable limit.

SUMMARY OF THE INVENTION According to the invention, the method relates to operating a grinding plant having a grinding mill with means for injecting cooling water into the mill. The mill also includes means for providing a controlled, ventilating air supply into the mill with an electrostatic precipitator means for removing dust and ground material from the air exiting from the mill. The improvement in the operation of the mill comprises starting the grinding mill while simultaneously maintaining the ventilating air supply at least at a minimum level until the temperature within the mill or of the ground product thereof is increased to the operating level of the mill. The method also comprises providing means to measure the temperature within the mill or of the ground product thereof and controlling the ventilating air supply in direct dependence upon at least one of the flow of cooling water and the rate of injection of cooling water into the mill such that initially increasing the air flow through the mill during starting is delayed, at least until reaching the operating temperature thereof and the operation of the plant is improved due to the interlocking dependence between the conditions at the input, the intermediate, and the output end portions which interlock, re sulting in timed coordination between the sections of the plant.

It has been found that by altering the normal starting procedure of a grinding plant in this manner, it is possible to obtain a substantially dust-free start-up. The air volume through the mill may be kept as low as possible by throttling the air by means of a damper or by adjusting the speed of the variable speed fan. Some air necessarily has to be drawn through the mill in order to establish and maintain a reduction in pressure throughout the mill proper, thereby preventing dust from escaping through the inlet or through sealings at the outlet or through casings enclosing conveyors leading therefrom. When the air flow is kept low, the velocity of the air passing through the plant is kept low as a result, and only small amounts of dust are carried by the air to the electrostatic precipitator. Under these circumstances, the electrostatic precipitator is capable of dedusting the air during starting even in the event that clouds of dust are produced and whirled up during the start.

When the temperature inside the mill rises above a certain predetermined level, cooling by means of internal water injection is begun. This cooling may be started by hand or it may be automatically started. Measurement of the temperature within the mill or of the product may be converted into a signal which is transmitted to, and capable of activating the cooling means.

The electrostatic precipitator contemplated as part of the invention is of the type having electrodes, such as emission and collection electrodes, in spaced relation with means for maintaining a voltage across the electrodes to create an electric field therebetween. Dust is thereby collected and is periodically removed there from preferably by shaking the electrodes at the appropriate frequency and amplitude while providing means for collecting the dust removed therefrom. The maximum voltage which can be maintained across the electrodes is generally a function of the temperature and the moisture content or dew point of the air between and around the electrodes. To operate the precipitator at a high efficiency, it is preferable to maintain a high voltage. However, this often results in electrical discharges across the plates which are known as flashover type discharges and can be monitored and interpreted as an indication of the actual voltage which is being maintained across the plates at a given time. Consequently, controlling of the voltage across the plates can be accomplished by controlling the rate of flash over-type discharges across the plates. A rapping means, which is generally in the form of a rapping gear, employs means to rap the electrodes periodically to cause the dust collected thereon to be removed and collected into the collecting system.

The method may include control of the performance of the electrostatic precipitator so that the air volume through the plant, as well as the voltage and shaking of collecting and discharge electrodes of the electrostatic precipitator during the initial starting up of the plant, are increased in dependence upon the cooling water flow rate.

When controlling the voltage of the precipitator and the shaking of the electrodes during starting up of the plant it is possible to adjust the performance of the electrostatic precipitator in accordance with the prevailing conditions in the plant, such as the temperature and the moisture content of the ventilating air.

The voltage of the electrostatic precipitator may be controlled in dependence upon the frequency of occurrence of flash-over in the electrostatic precipitator.

It is important that the efficiency of the electrostatic precipitator is as high as possible, particularly during starting up of the grinding plant. Since the potential difference across the electrodes of the precipitator is an important factor influencing the efficiency of the precipitator and since this voltage should be as high as possible, it is apparent that the rate of occurrence of flashover will also be substantial. However, during starting up of the plant, a higher frequency of flash-overs may be tolerated in order to obtain optimum performance of the precipitator. Controlling the voltage of the electrostatic precipitator by controlling the frequency of occurrence of flash-overs is a permissible method of obtaining such high performance.

Another feature of the invention relates to the control of the shaking of the electrodes comprising controlling of the amplitude as well as of the frequency of the shaking of the electrodes.

When shaking the electrodes, dust accumulated on them drops down and may under unfavorable conditions give rise to clouds of dust puffing out of the precipitator. To avoid the formation of such clouds, which the precipitator may have difficulties in coping with during the starting operation, it is important to control the amplitude and the frequency of the shaking of the electrodes at a level in dependence on the other factors influencing the performance of the precipitator. This is particularly so when the clouds are formed at the outlet end of the precipitator.

In some grinding plants more than one dedusting unit is installed. For example, part of the conveyors may often be dedusted by drawing air to a separate dedusting unit. During starting up of such plants the auxiliary dedusting unit is loaded with modest amounts of air only and part of the air passing through the mill during the initial starting up of the plant may by-pass the electrostatic precipitator and may be cleaned in the separate dedusting unit, thus keeping the load on the electrostatic precipitator at a low level to avoid the electrostatic precipitator being overloaded during the starting The invention includes a plant for carrying out the method, the plant including a grinding mill having cooling water injection equipment, and a ventilating air supply with an electrostatic precipitator control means for varying the flow rate of air ventilating the plant, the control means being operable in response to at least one of the rate of flow of cooling water and the dew point measurements taken from the air exiting from the mill.

The means controlling the air volume passing through the plant is linked with the automatic equipment which starts the internal water injection. The equipment may comprise interlocking and time-lag relays working in dependence on the start of other units of the plant, or on temperature or dew point measurements. The water injection, and consequently the control of the air volume, may be regulated in increasing amounts in dependence upon the heat developed during the starting up of the plant. The air volume control means may include a throttling damper.

Alternatively, the air volume control means may comprise a variable speed air ventilating fan. The amount of air drawn through the mill is kept moderately low so that the amount of dust which may be entrained in the air during its passage through the plant is low enough for the electrostatic precipitator to separate off all or nearly all the dust even under the unfavorable conditions prevailing in the precipitator during starting up.

The voltage and the shaking of the electrodes may be controlled in accordance with the prevailing conditions and with the amounts of air to be dedusted.

The control equipment for starting up the plant may include means arranged to count the number of flashovers in the electrostatic precipitator and means to adjust the voltage and the shaking of the electrodes accordingly.

A stepwise or continuous increase or decrease in the current is under normal conditions a suitable means for controlling the precipitator in dependence upon the dew point of the air passing through the plant. The working of the precipitator is, in this manner, correlated with the actual conditions, thereby obtaining a near perfect performance of the precipitator. This control involves rather complicated and delicate equipment which also needs regular and frequent maintenance. However, by introducing the numbers of flashovers occurring in the electrostatic precipitator as an indication of the condition of the air, an adequate and reliable control of the performance of the precipitator may be achieved.

In addition to the measures taken during starting up of the grinding plant, further measures taken during stopping may also improve the starting conditions. Thus, the control means may include apparatus arranged to maintain automatic operation of the electrostatic precipitator for a predetermined time after the grinding mill has ceased to operate.

By making provision for an additional running period of the electrostatic precipitator the settling or accumulation of dust in parts of the grinding plant may be prevented and the plant may be more or less cleaned off. The dust problems during starting up of the plant are thereby reduced to a minimum and, therefore, arise largely from the dust produced during the initial starting up. This dust is easily separated off by the precipitator when the air volume is controlled, even though the starting conditions for the electrostatic precipitator are unfavorable.

In cases where the grinding plant has an auxiliary dedusting unit, it is an advantage to use this unit during starting up of the plant. Accordingly, a branch pipe may be connected from a point between the grinding mill and the electrostatic precipitator, to this auxiliary dedusting unit, with means to vary air flow through the branch pipe in dependence upon the cooling water flow rate.

BRIEF DESCRIPTION OF THE DRAWING A preferred embodiment of the invention is described hereinbelow with reference to the drawing wherein:

The sole FIGURE is a preferred grinding plant of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description I refer to Interlock and lnterlocking" as an arrangement whereby the operation of one part of mechanism automatically brings about or prevents the operation of another in accordance with a prescribed sequence of operational steps.

Referring to the drawing there is shown the main machinery of a cement grinding plant for the straightgrinding of cement clinker. The plant comprises a tube mill 1 having an inlet box 2 for feeding material to the mill through a hollow trunnion 3 and an outlet box 4 for receiving ground material discharged from the interior of the mill through a hollow outlet trunnion S. The material to be ground in the mill is fed to the inlet box 2 by means of one or more weighing feeders 6 which extract the material from one or more hoppers 7.

The outlet box 4 communicates through a pipe 8 with a screw conveyor 9 having a discharge opening 10 for discharging the ground material. The outlet box 4 also communicates through a pipe 11 with an electrostatic precipitator 12. The electrostatic precipitator 12 has a bottom hopper 13 with an extraction conveyor 14 feeding the dust separated off by the electrostatic precipitator into the screw conveyor 9 through a pipe 15. The cleaned air passes through an outlet 16 having a throttling damper 17 and a fan 18, from which the air passes to a vent 19 or a chimney.

The tube mill is driven by an electric motor (not shown) which is connected through a gear (not shown) to the mill via a shaft 20. The shaft 20 is connected to the trunnion 5. The shaft 20 is provided with a stuffing box 21, through which water from a stationary water pipe 22 may be transferred to a water injection installation 23 passing through the shaft to the interior of the mill 1. The water pipe 22 is connected to a suitable source (not shown) delivering water and compressed air to the pipe. The temperature of the ground product is measured by means of a temperature measuring device 24. Another temperature measuring device is provided in the outlet 16 for measuring the air temperature. A dew point measuring device 26 is arranged in the outlet for measurement of the dew point of the exiting air. The signals provided by temperature measuring devices 24 and 25 and the signal from dew point measuring devices 26 are monitored either for manual or for automatic interlocked operation.

The pipe 11 has a branch pipe 27 with a damper 28 through which part of the air may by-pass the electrostatic precipitator 12 for dedusting in other dedusting equipment, not shown in the drawing, similar to the equipment shown, but in addition thereto, and remotely positioned in a known manner.

In operation, material to be ground in the mill is fed to the mill 1 by means of the weighing feeders 6 and is passed through the hollow trunnion 3 to the interior of the mill 1 where it is ground to the required fineness such as by means of a charge of grinding bodies in the mill.

During grinding of the material heat is developed inside the mill and the temperature rises. However, in order not to damage the product, such as by dehydration of the gypsum, the temperature is kept below specified limits by injecting water into the mill. The water injection is controlled by the temperature of the ground product, which is measured by the temperature measuring device 24. The temperature is then converted into a suitable signal which is transmitted to the servomotor S connected to the Value V to control the supply of water from the water injection into the grinding mill when the temperature of the ground product reaches a predetermined level requiring the introduction of cooling water as monitored by measuring device 24. The water injected may consist of a constant amount of water plus a minor adjustable amount, so that only the adjustable amount is controlled by the temperature when the plant is in full operation. Alternatively, it may consist of a continuously variable supply of water. The product is carried by means of the conveyor 9 to the discharge opening 10 from where it is carried to a silo or a classifier.

The mill is ventilated by means of the variable speed air ventilating fan 18 which maintains a reduced pressure in the mill in order to avoid dust nuisances from the inlet and outlet boxes. The ventilating air passes through the electrostatic precipitator 12 primarily in order to be cleaned so that the air leaving the plant through the vent 19 is substantially clean. A second reason for cleaning the air leaving the plant is to recover for future use the ground material and the dust carried out of the mill suspended in the air. The throttling damper 17 controls the air volume passing through the plant and thereby the pressure. Thus the ventilating air may be controlled exclusively by either the variable speed ventilating fan 18 or the throttle damper 17, but is preferably controlled by both devices in combination with the central control computer and rectifier. For example, the temperature measurements of the ground product are taken by device 24 and transmitted to the control computer rectifier. The computer in turn, transmits a suitable signal to the variable speed ventilating fan motor 20, as well as a signal to the servomotor S which controls the position of throttling damper 17. These signals control the ventilating air supply to the grinding mill 1 in dependence upon either the temperature measurement of the ground product as transmitted by device 24 or the dew point measurement transmitted from device 26. The signals from devices 24 and 26 are transmitted by their respective relays (appropriately denoted R) to the central control computer rectifier.

The various components and measuring devices, including temperature measuring devices 24, 25 and 26, throttle damper l7, electrostatic precipitator 12, variable speed fan 18, water injection equipment 21, valve V, and water pipe 22, etc., are connected in interlocked circuitry of the known type having relays and servomotors, with a central control computer and a rectifier as shown schematically in the drawing and as described previously. This circuitry processes the signals which indicate the conditions at the input portion, the grinding mill portion and the output portion of the plant, which signals are fed to the central control computer rectifier to control the conditions in the plant.

The starting operation of the plant can be more clearly understood by referring to the following sequence of steps which illustrates the interlocking ar- INTERLOCKING SEQUENCE Damper position 1 Rectifier for precipitator started position 1 Conveyors started Air fan started Mill started Mill feeders started Rapping of precipitator started Rectifier for precipitator started position 2 Water injection started Damper position 2 Rectifier for precipitator started position 3 Damper position 3 7) (i2) J4) (18) (6) 2) 7) (l 7) The throttling damper 17 has to be in a nearly closed position (position 1) to keep the air volume low before any part of the plant can be started. This is preferably automatically controlled with the start up of the mill or it may be accomplished manually. The air volume passing through the mill should be as low as possible, but the damper cannot-be closed completely because some air has to be passed through the mill in order to create a partial vacuum at the material feed end of the mill to prevent dust clouds which are raised by the tumbling over of the grinding bodies during the initial starting up of the mill, from passing out of the feed end. The start position for the throttling damper is determined by these conditions. When this requirement is fulfilled, the plant may be started in the sequence indicated. Firstly, the precipitator (i.e. the rectifier for the precipitator) is started. However, the current to the precipitator is kept at a minimum (position l.) and is blocked automatically by the prescribed interlocking sequence. For manual operation it is manually prevented from activation until the appropriate time in the sequence. Simultaneously, or immediately thereafter, the conveyors and the air ventilating fan are started. With some time lag the mill is started and immediately thereafter the feeders to the mill, and the rapping mechanism for the precipitator are started.

Once the grinding in the mill is in operation, heat is developed during the grinding. The temperature is sensed by the temperature measuring equipment 24 which automatically switches the rectifier for the electrostatic precipitator to a higher voltage (position 2) adjusted to the new condition. This preferably occurs when the product reaches a temperature of approximately 90C. When the temperature reaches 1 C the water injection equipment is started and at the same time the draft through the mill is increased by changing the damper to a more open position (position 2).

After some time (in this case a few minutes) the running of the plant is stabilized with all temperature measurements being maintained within the desired predetermined temperature ranges and the rectifier is switched over to the highest voltage position (position 3) and the damper is released to position 3 for a handoperated or preferably an automatic control.

As mentioned above the plant may be combined with a classifier for classifying the product discharged 8 through the outlet 10 and returning the coarse fraction to the mill for further grinding.

The temperature measurements used in controlling the plant may be taken at various locations within the grinding mill and at the outlet portion for manually or automatically controlling the conditions in the plant, and the water injection may be controlled continuously or intermittently.

I claim:

l. A method of operating a grinding plant having a grinding mill with means for introducing cooling water into the mill, means for providing a controlled ventilating air supply into the mill for ventilating the mill, an electrostatic precipitator means for removing dust from the air exiting from the mill, the improvement which comprises starting said grinding mill while simultaneously maintaining said ventilating air supply at a minimum level until the temperature of the ground product of the mill is increased to a predetermined level requiring the introduction of cooling water to maintain the temperature of the ground product at least at said predetermined level, activating the flow of cooling water into the mill when said predetermined temperature level of said ground product is reached, providing temperature sensing means to measure the temperature of the ground product of the mill, and controlling said ventilating air supply in direct dependence upon the flow of cooling water into said mill such that the temperature levels of the mill are thereby controlled by the air flow and the water introduced therein.

2. The method according to claim 1 further comprising increasing the voltage across said precipitator in dependence upon the rate of flow of cooling water into said mill and controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.

3. A method according to claim 1 further comprising controlling the voltage across said dust precipitator by measuring the frequency of flash-over-type electrical discharges across said precipitator and controlling the flash-over-type discharges across said precipitator in dependence upon the rate of flow of cooling water to said mill such that the frequency of flash-over-type discharges and the voltage is thereby dependent upon, and interrelated to, the rate of flow of cooling water to said mill.

4. The method according to claim 3 further comprising controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.

5. The method according to claim 1 further comprising controlling the ventilating air entering into said mill by throttling the ventilating air exiting from said mill.

6. The method of operating a grinding plant according to claim 1 further comprising providing interlocking circuitry with a central control means and controlling the temperature levels of the mill by the air flow and the water introduced therein in dependence upon the temperature levels of the ground product thereof.

7. A method of operating a grinding plant having a grinding mill with means for injecting cooling water into the mill, means for providing a controlled ventilating air supply into the mill for ventilating the mill, an electrostatic precipitator means for removing dust from the air exiting from the mill, the improvement which comprises starting said grinding mill while simultaneously maintaining said ventilating air supply at least at a minimum level until the temperature of the ground product thereof is increased to a predetermined level requiring the introduction of cooling water to maintain the temperature of the ground product at least at said predetermined level, providing dew point sensing means to measure the dew point in the air exiting from said precipitator and controlling said ventilating air supply to said mill in dependence upon said dew point measurements, such that initially increasing the air flow through said mill during starting is delayed at least until the temperature of the ground product reaches said predetermined level and the temperature levels of the ground product are controlled by the dew point measurements at the output of the mill, the air flow therethrough and the water introduced therein.

8. The method according to claim 7 further comprising increasing the voltage across said precipitator in dependence upon the rate of flow of cooling water into said mill and controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.

9. A method according to claim 7 further comprising controlling the voltage across said dust precipitator by measuring the frequency of flash-over-type electrical discharges across said precipitator and controlling the flash-over-type discharges across said precipitator in dependence upon the rate of flow of cooling water to said mill such that the frequency of flash-over-type discharges and the voltage is thereby dependent upon, and interrelated to, the rate of flow of cooling water to said mill.

10. The method according to claim 9 further comprising controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.

11. The method according to claim 7 further comprising controlling the ventilating air entering into said mill by throttling the ventilating air exiting from said mill.

12. The method of operating a grinding plant according to claim 7 further comprising providing interlocking circuitry with a central control means and controlling the temperature levels of the mill by the air flow and the water introduced therein in dependence upon the temperature levels of the ground product thereof.

13. A plant for grinding materials which comprises a grinding mill, a supply of cooling water for cooling said mill with means for introducing said cooling water into said mill, means for providing a ventilating air supply to said mill for ventilating the mill, with at least one electrostatic dust precipitator means, means for measuring the dew point of the air exiting from the mill, means for controlling said ventilating air supply in direct dependency upon at least one of the rate of flow of cooling water, the rate of introduction of cooling water and dew point measurements taken from the air exiting from the precipitator, and means to maintain the ventilating air supply at a minimum level until the temperature of the ground product is increased to a predetermined level necessitating the introduction of cooling water such that the temperature levels of the mill are controlled by the air flow and water introduced therein and the dew point of the air exiting therefrom.

14. The plant according to claim 13 further comprising means for collecting the dust removed from said Ed electrostatic precipitator, and said air ventilating control means being operationally connected to control the voltage across the electrostatic precipitator.

15. The plant according to claim 14 wherein the control means is capable of counting the number of flashover-type electrical discharges across said electrostatic precipitator and said control means is adapted to control the voltage across said electrostatic precipitator in relation to the prevailing conditions in the plant by controlling the frequency of flash-over-type discharges across said electrostatic precipitator.

16. The plant according to claim 15 wherein the control means further comprises means to maintain auto matic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

17. The plant according to claim 14 wherein the (102% trol means further comprises means to maintain ants matic operation of said electrostatic precipitate" least for an additional time period after the grin mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

18. The plant according to claim It? wherein a control means for varying the flow rate of air ventilating the plant comprises a throttling damper.

19. The plant according to claim 18 further comprising means for collecting the dust removed from said electrostatic precipitator, and said air ventilating con.- trol means being operationally connected to control. the voltage across the electrostatic precipitator.

20. The plant according to claim 19 wherein the control means is capable of counting the number of flash over-type electrical discharges across said electrostatic precipitator and said control means is adapted to control the voltage across said electrostatic precipitator in relation to the prevailing conditions in the plant by controlling the frequency of flash-over-type discharges across said electrostatic precipitator.

21. The plant according to claim 20 wherein the con trol means further comprises means to maintain automatic oepration of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

22. The plant according to claim 19 wherein the ccntrol means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

23. The plant according to claim 18 wherein the con trol means further comprises means to maintain auto matic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

24. The plant according to claim l3 wherein said control means for varying the plant comprises a variable speed air ventilating fan,

25. The plant according to claim 24 further comprising means for collecting the dust removed from said electrostatic precipitator, and said air ventilating con- 1 l trol means being operationally connected to control the voltage across the electrostatic precipitator.

26. The plant according to claim 25 wherein the control means is capable of counting the number of flashover-type electrical discharges across said electrostatic precipitator and said control means is adapted to control the voltage across said electrostatic precipitator in relation to the prevailing conditions in the plant by controlling the frequency of flash-over-type discharges across said electrostatic precipitator.

27. The plant according to claim 26 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

28. The plant according to claim 25 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

29. The plant according to claim 24 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.

30. The plant according to claim 13 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator,

31. The plant according to claim 13 further comprising a branch pipe connected at a location between the grinding mill and the electrostatic precipitator, a second means for removing dust from the air exiting from the mill, and means to control the air flow through said pipe in dependence upon the rate of flow of cooling water to said mill such that the relation between the rate of flow of cooling water and the rate of flow of ventilating air through said second pipe is interlocked with resulting improved starting and operational performance.

32. A plant for grinding materials such as cement clinker which comprises a grinding mill, a supply of cooling water for cooling said mill, means for controlling said supply of cooling water to said mill, means for providing a ventilating air supply to said mill, at least one electrostatic precipitator, at least one electrostatic precipitator control means for varying the flow rate of air ventilating the plant, said control means being operably connected to the means for controlling the flow of cooling water to said mill in a manner such that the ventilating air supply is directly dependent upon the flow of cooling water to said mill, means for maintaining a maximum voltage across said electrostatic precipitator in relation to at least one of the temperatures within the mill and the prevailing temperature and moisture conditions of the air exiting from the plant, means for operating said electrostatic precipitator at maximum levels, and means to collect the dust removed from said precipitator such that said precipitator is capable of being operated at maximum efficiency in relation to the prevailing temperature and moisture conditions in the plant to thereby reduce the dust content emitted from the plant to minimum levels and to control the conditions at the input portion, the grinding mill, and the output portions of the plant by the ventilating air supply, the water supply and the electrostatic precipitator operation of said mill. =4: i: a is 

1. A METHOD OF OPERATING A GRINDING PLANT HAVING A GRIDING MILL WITH MEANS FOR INTRODUCING COOLING WATER INTO THE MILL, MEANS FOR PROVIDING A CONTROLLED VENTILATING AIR SUPPLY INTO THE MILL FOR VENTILATING THE MILL, AN ELECROSTATIC PRECIPITATOR MEANS FOR REMOVING DUST FROM THE AIR EXITING FROM THE MILL, THE IMPROVEMENT WHICH COMPRISES STARTING SAID GRINDING MILL WHILE SIMULTANEOUSLY MAINTAINING SAID VENTILATING AIR SUPPLY AT A MINIMUM LEVEL UNTIL THE TEMPERATURE OF THE GROUND PRODUCT OF THE MILL IS INCREASE TO A PREDETERMINED LEVEL REQUIRING THE INTRODUCTION OF COOLING WATER TO MAINTAIN THE TEMPERATURE OF THE GROUND PRODUCT AT LEAST AT SAID PREDETERMINED LEVEL, ACTIVATING THE FLOW OF COOLING WATER INTO THE MILL WHEN SAID PREDETERMINED TEMPERATURE LEVEL OF SAID GROUND PRODUCT IS REACHED, PROVIDING TEMPERATURE SENSING MEANS TO MEASURE THE TEMPERATURE OF THE GROUND PRODUCT OF THE MILL, AND CONTROLLING SAID VENTILATING AIR SUPPLY IN DIRECT DEPENDENCE UPON THE FLOW OF COOLING WATER INTO SAID MILL SUCH THAT THE TEMPERATURE LEVELS OF THE MILL ARE THEREBY CONTROLLED BY THE AIR FLOW AND THE WATER INTRODUCED THEREIN.
 2. The method according to claim 1 further comprising increasing the voltage across said precipitator in dependence upon the rate of flow of cooling water into said mill and controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.
 3. A method according to claim 1 further comprising controlling the voltage across said dust precipitator by measuring the frequency of flash-over-type electrical discharges across said precipitator and controlling the flash-over-type discharges across said precipitator in dependence upon the rate of flow of cooling water to said mill such that the frequency of flash-over-type discharges and the voltage is thereby dependent upon, and interrelated to, the rate of flow of cooling water to said mill.
 4. The method according to claim 3 further comprising controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.
 5. The method according to claim 1 further comprising controlling the ventilating air entering into said mill by throttling the ventilating air exiting from said mill.
 6. The method of operating a grinding plant according to claim 1 further comprising providing interlocking circuitry with a central control means and controlling the temperature levels of the mill by the air flow and the water introduced therein in dependence upon the temperature levels of the ground product thereof.
 7. A method of operating a grinding plant having a grinding mill with means for injecting cooling water into the mill, means for providing a controlled ventilating air supply into the mill for ventilating the mill, an electrostatic precipitator means for removing dust from the air exiting from the mill, the improvement which comprises starting said grinding mill while simultaneously maintaining said ventilating air supply at least at a minimum level until the temperature of the ground product thereof is increased to a predetermined level requiring the introduction of cooling water to maintain the temperature of the ground product at least at said predetermined level, providing dew point sensing means to measure the dew point in the air exiting from said precipitator and controlling said ventilating air supply to said mill in dependence upon said dew point measurements, such that initially increasing the air flow through said mill during starting is delayed at least until the temperature of the ground product reaches said predetermined level and the temperature levels of the ground product are controlled by the dew point measurements at the output of the mill, the air flow therethrough and the water introduced therein.
 8. The method according to claim 7 further comprising increasing the voltage across said precipitator in dependence upon the rate of flow of cooling water into said mill and controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.
 9. A method according to claim 7 further comprising controlling the voltage across said dust precipitator by measuring the frequency of flash-over-type electrical discharges across said precipitator and controlling the flash-over-type discharges across said precipitator in dependence upon the rate of flow of cooling water to said mill such that the frequency of flash-over-type discharges and the voltage is thereby dependent upon, and interrelated to, the rate of flow of cooling water to said mill.
 10. The method according to claim 9 further comprising controlling the rate of collection of dust precipitated by said precipitator from the air exiting from said mill in dependence upon the flow of air through said mill.
 11. The method according to claim 7 further comprising controlling the ventilating air entering into said mill by throttling the ventilating air exiting from said mill.
 12. The method of operating a grinding plant according to claim 7 further comprising providing interlocking circuitry with a central control means and controlling the temperature levels of the mill by the air flow and the water introduced therein in dependence upon the temperature levels of the ground product thereof.
 13. A plant for grinding materials which comprises a grinding mill, a supply of cooling water for cooling said mill with means for introducing said cooling water into said mill, means for providing a ventilating air supply to said mill for ventilating the mill, with at least one electrostatic dust precipitator means, means for measuring the dew point of the air exiting from the mill, means for controlling said ventilating air supply in direct dependency upon at least one of the rate of flow of cooling water, the rate of introduction of cooling water and dew point measurements taken from the air exiting from the precipitator, and means to maintain the ventilating air supply at a minimum level until the temperature of the ground product is increased to a predetermined level necessitating the introduction of cooling water such that the temperature levels of the mill are controlled by the air flow and water introduced therein and the dew point of the air exiting therefrom.
 14. The plant according to claim 13 further comprising means for collecting the dust removed from said electrostatic precipitator, and said air ventilating control means being operationally connected to control the voltage across the electrostatic precipitator.
 15. The plant according to claim 14 wherein the control means is capable of counting the number of flash-over-type electrical discharges across said electrostatic precipitator and said control means is adapted to control the voltage across said electrostatic precipitator in relation to the prevailing conditions in the plant by controlling the frequency of flash-over-type discharges across said electrostatic precipitator.
 16. The plant according to claim 15 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 17. The plant according to claim 14 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 18. The plant according to claim 13 wherein said control means for varying the flow rate of air ventilating the plant comprises a throttling damper.
 19. The plant according to claim 18 further comprising means for collecting the dust removed from said electrostatic precipitator, and said air ventilating control means being operationally connected to control the voltage across the electrostatic precipitator.
 20. The plant according to claim 19 wherein the control means is capable of counting the number of flash-over-type electrical discharges across said electrostatic precipitator and said control means is adapted to control the voltage across said electrostatic precipitator in relation to the prevailing conditions in the plant by controlling the frequency of flash-over-type discharges across said electrostatic precipitator.
 21. The plant according to claim 20 wherein the control means further comprises means to maintain automatic oepration of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 22. The plant according to claim 19 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 23. The plant according to claim 18 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 24. The plant according to claim 13 wherein said control means for varying the plant comprises a variable speed air ventilating fan.
 25. The plant according to claim 24 further comprising means for collecting the dust removed from said electrostatic precipitator, and said air ventilating control means being operationally connected to control the voltage across the electrostatic precipitator.
 26. The plant according tO claim 25 wherein the control means is capable of counting the number of flash-over-type electrical discharges across said electrostatic precipitator and said control means is adapted to control the voltage across said electrostatic precipitator in relation to the prevailing conditions in the plant by controlling the frequency of flash-over-type discharges across said electrostatic precipitator.
 27. The plant according to claim 26 wherein the control means further comprises means fo maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 28. The plant according to claim 25 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 29. The plant according to claim 24 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 30. The plant according to claim 13 wherein the control means further comprises means to maintain automatic operation of said electrostatic precipitator at least for an additional time period after the grinding mill has ceased to operate such that dust remaining in the plant will be substantially removed prior to ceasing operation of said precipitator.
 31. The plant according to claim 13 further comprising a branch pipe connected at a location between the grinding mill and the electrostatic precipitator, a second means for removing dust from the air exiting from the mill, and means to control the air flow through said pipe in dependence upon the rate of flow of cooling water to said mill such that the relation between the rate of flow of cooling water and the rate of flow of ventilating air through said second pipe is interlocked with resulting improved starting and operational performance.
 32. A plant for grinding materials such as cement clinker which comprises a grinding mill, a supply of cooling water for cooling said mill, means for controlling said supply of cooling water to said mill, means for providing a ventilating air supply to said mill, at least one electrostatic precipitator, at least one electrostatic precipitator control means for varying the flow rate of air ventilating the plant, said control means being operably connected to the means for controlling the flow of cooling water to said mill in a manner such that the ventilating air supply is directly dependent upon the flow of cooling water to said mill, means for maintaining a maximum voltage across said electrostatic precipitator in relation to at least one of the temperatures within the mill and the prevailing temperature and moisture conditions of the air exiting from the plant, means for operating said electrostatic precipitator at maximum levels, and means to collect the dust removed from said precipitator such that said precipitator is capable of being operated at maximum efficiency in relation to the prevailing temperature and moisture conditions in the plant to thereby reduce the dust content emitted from the plant to minimum levels and to control the conditions at the input portion, the grinding mill, and the output portions of the plant by the ventilating air supply, the water supply and the electrostatic precipitator operation of said mill. 